The invention relates generally to a device and to a corresponding method for reading out X-ray information stored in a storage phosphor layer.
X-ray pictures can be stored in so-called storage phosphors, whereby the X-ray radiation passing through an object, for example a patient, is stored as a latent picture in a storage phosphor layer. In order to read out the latent picture, the storage phosphor layer is irradiated with stimulation light, and so stimulated into emitting emission light. The emission light, the intensity of which corresponds to the picture stored in the storage phosphor layer, is collected by an optical detector and converted into electric signals. The electric signals are further processed, as required, and finally made available for examination, in particular for medical diagnostic purposes, whereby they are displayed in a corresponding display unit, such as a monitor or a printer.
In U.S. Pat. No. 4,484,073, herein incorporated by reference in its entirety for background information only, a device and a method are described whereby a laser beam is deflected by means of a galvanometer mirror in such a way that the laser beam hits the storage phosphor layer to be read out essentially in the form of a dot, and passes over said storage phosphor layer in a linear area. The emission light emitted here by the storage phosphor layer is collected by a photomultiplier and converted into an electric signal. By conveying the storage phosphor layer perpendicularly to the linear area, individual linear areas of the storage phosphor layer can be successively read out so that finally, a two-dimensional X-ray picture is obtained which is made up from individual lines which are, in turn, respectively made up from a number of individual pixels. In order to convert the electric signal of the photomultiplier into individual picture signals allocated to the respective pixels of the X-ray picture, a position reference pulse is created for each pixel of a line by means of a so-called linear encoder, and the electric signal of the photomultiplier is integrated respectively in accordance with a position reference pulse over a fixed time interval. The signal value obtained with the respective integration is allocated to the corresponding pixel.
Because the speed of the deflection element is generally subject to fluctuations, the speed of the laser beam is not constant over the whole line to be read out, i.e., within the predetermined fixed time interval, the laser beam passes over areas of the storage phosphor layer which are of different widths in accordance with a position reference pulse depending upon the speed at that moment. This means that, with the described integration of the electric signal from the photomultiplier, areas of different widths are generally collected. A high momentary speed of the laser beam thus means that a relatively wide section of the area corresponding to a pixel is scanned by the laser beam in the pre-specified time interval. On the other hand, a low momentary speed leads to collection of a relatively narrow section of the area of the line allotted to a pixel. Overall, a partial loss of information and so a negative effect upon the picture quality of the X-ray picture read out must therefore be accepted.
It is the aim of the invention to provide a device and a corresponding method for reading out X-ray information stored in a storage phosphor layer with which the picture quality is improved, in particular with a reduction of information loss.